Having a problom with Threads

Question

I'm trying to send a random PictureBox to the timer1_tick but I'm having a problem with it...

This is my code by now.

 

using System;

using System.Collections.Generic;

using System.ComponentModel;

using System.Data;

using System.Drawing;

using System.Linq;

using System.Text;

using System.Threading.Tasks;

using System.Windows.Forms;

using System.Threading;

using System.IO;

using System.Drawing.Imaging;

using System.Drawing.Drawing2D;

using System.Collections;

using System.Globalization;

using System.Resources;

namespace Move_the_Block_Project

{

    public partial class Form1 : Form

    {

        public Form1()

       {

            InitializeComponent();

            PictureBox Arrow_S;

            int n;

            List Arrows = new List(3);

           Arrows.Add(A_Up);

           Arrows.Add(A_Right);

           Arrows.Add(A_Left);

           Arrows.Add(A_Down);

           n = SelectRandom();

           Arrow_S = Arrows[n];

           timer1.Interval = 10;

        }

        public int SelectRandom()

        {

            Random random = new Random();

            int a = random.Next(0, 3);

            return a;

         }

        private void timer1_Tick(object sender, EventArgs e)

        {

            Arrow_S = ????

            Arrow_S.Location = new Point(Arrow_S.Location.X, Arrow_S.Location.Y + 1);

        }

        private void Start_B_Click(object sender, EventArgs e)

        {

            bool press= true;

            if (press == true)

           {

                timer1.Start();

           }

          else

          {

                timer1.Stop();

          }

        }

    }

}